Founded in 1839, Williamson County is one of 102 counties in the state of Illinois. Historically, Williamson County was formed from Franklin County. The city of Marion is the county seat. With FIPS code of 199, Williamson County has a population of 66,879 and area of 420 square miles (1,088 km2).
List of all cities in Williamson County, IL alphabetically
| City/Town | Postal Code | Area Code |
| Attila, IL | 62974 | 618/730 |
| Blairsville, IL | 62918 | 618/730 |
| Cambria, IL | 62915 | 618/730 |
| Carterville, IL | 62918 | 618/730 |
| Colp, IL | 62921 | 618/730 |
| Crainville, IL | 62918 | 618/730 |
| Creal Springs, IL | 62922 | 618/730 |
| Dewmaine, IL | 62918 | 618/730 |
| Egyptian Hills, IL | 62922 | 618/730 |
| Egyptian Shores, IL | 62922 | 618/730 |
| Energy, IL | 62933 | 618/730 |
| Ferges, IL | 62951 | 618/730 |
| Fergestown, IL | 62951 | 618/730 |
| Freeman Spur, IL | 62841 | 618/730 |
| Hafer, IL | 62918 | 618/730 |
| Herrin, IL | 62948 | 618/730 |
| Hurst, IL | 62949 | 618/730 |
| Johnston City, IL | 62951 | 618/730 |
| Lake Crest, IL | 62922 | 618/730 |
| Marion, IL | 62959 | 618/730 |
| New Camp, IL | 62921 | 618/730 |
| Old Camp, IL | 62921 | 618/730 |
| Paineville, IL | 62948 | 618/730 |
| Pittsburg, IL | 62974 | 618/730 |
| Shakerag, IL | 62951 | 618/730 |